3GPP-LTE (3rd Generation Partnership Project-Long Term Evolution) has discussed a transmission method for uplink control channels in different two ways: “in a case in which uplink control signals and uplink data are transmitted simultaneously”; and “in a case in which uplink control signals and uplink data are not transmitted simultaneously.”
When uplink control signals and uplink data are transmitted simultaneously, preferably, control signals are transmitted in synchronization with data using uplink resources designated by the base station. Meanwhile, when uplink data signals are not permitted to be transmitted and therefore uplink control signals are not transmitted in synchronization with uplink data, terminals transmit uplink control signals using “a band for transmitting uplink control signals” reserved in advance.
A band (PUCCH: Physical Uplink Control Channel) that is reserved for transmitting uplink control signals (e.g. ACK/NACKs and CQIs) by 3GPP-LTE is shown in FIG. 1. In FIG. 1, the vertical axis represents the system bandwidth of which values unique to the base station, for example, 5 MHz or 10 MHz are set, and the horizontal axis represents time. One subframe length is 1 ms, and PUCCH transmission is performed per subframe. In addition, one subframe is composed of two slots. As shown in FIG. 1, frequency resources allocated to control signals are frequency-hopped at the time slots are switched, so that it is possible to obtain the frequency diversity effect.
Moreover, FIG. 2 is a drawing conceptually showing a state in which terminals transmit CQIs using a band reserved by the system. Here, each ZAC sequence in the figure has a sequence length of twelve in the time domain, and has a characteristic of constant Amplitude (CA) in the frequency domain and the characteristic of zero auto correlation (ZAC) in the time domain.
Each slot of a subframe for transmitting CQIs is formed by seven SC-FDMA (Single Carrier-Frequency Division Multiple Access) symbols. Hereinafter SC-FDMA symbols in a slot are referred to as the first, second, seventh SC-FDMA symbols. CQI signals are placed in the first, third, fourth, fifth and seventh SC-FDMA symbols and reference signals (RSs) for demodulating CQIs are placed in the second and sixth SC-FDMA symbols. As shown in FIG. 2, each of five CQI symbols is primarily spread by a ZAC sequence in the frequency domain, and placed in a SC-FDMA symbol (or “LB”: Long Block). In addition, reference signals obtained by performing the IFFT (Inverse Fast Fourier Transform) of ZAC sequences represented in the frequency domain are placed in the second and sixth SC-FDMA symbols.
ZAC sequences and amounts of cyclic shift used in each terminal are determined according to commands from the base station. Here, although cyclic shifting means transforming the waveform of ZAC sequences transformed in the time domain using cyclic shifting, an equivalent processing is possible by phase rotation in the frequency domain, so that a state in which cyclic shift processing is performed in the frequency domain is shown here. In addition, it has been determined that CQIs from different terminals are code-multiplexed (CDM). To be more specific, CQI signals from different terminals are transmitted through the same ZAC sequences having different amounts of cyclic shift. On the base station side, it is possible to separate CQI signals from terminals by taking into account of the amount of cyclic shift per terminal after correlation processing with ZAC sequences. That is, CQIs from different terminals are code-multiplexed.
In addition, 3GPP-LTE has determined that, when one terminal transmits CQIs and response signals (ACK/NACKs) simultaneously, response signals may be transmitted using reference signals for demodulating CQIs. The details are described later.
FIG. 3 is a drawing showing the characteristic of ZAC sequences used to primarily spread CQIs in the time domain. Each ZAC sequence has a sequence length of twelve in the time domain, and therefore there are maximum twelve patterns of cyclic shift. Since the cross-correlation between the same ZAC sequences having different amounts of cyclic shift is approximately zero, it is possible to separate signals spread through the same ZAC sequences having different amounts of cyclic shift in the time domain almost without interference.
However, although in an ideal environment as shown in FIG. 3, it is possible to separate signals spread by means of ZAC sequences with different amounts of cyclic shift without interference from each other by correlation processing on the receiver side, those signals do not necessarily reach the base station side simultaneously, due to the influence of channel delay, difference between timings terminals transmit signals, frequency offset and so forth. By this influence of timing difference, for example, as shown in FIG. 4, separation characteristics of signals spread by sequences corresponding to adjacent cyclic shifts are likely to deteriorate. In addition, the difference in transmission timings of terminals exerts a negative influence on the orthogonality between adjacent cyclic shifts of ZAC sequences. For example, in FIG. 3, assuming that amounts of cyclic shift obtained by shifting sequences one by one (twelve sequences of cyclic shift indexes i=0 to 12) is allocated to each terminal, it is possible to multiplex maximum twelve terminals according to differences in the amount of cyclic shift. That is, it is possible to code-multiplex twelve CQI signals using one frequency resource.
Methods of transmitting CQIs in a PUCCH field reserved for transmitting control information are described in non-patent documents 1 to 3. With Non-Patent document 1, when only CQIs are transmitted, the phase difference between two reference signals in a slot is fixed regardless of the amount of cyclic shift as shown in FIG. 2.
In addition, with non-patent documents 2 and 3, when CQIs and response signals are transmitted simultaneously, response signals are represented by multiplying CQI demodulating reference signals by complex coefficients {w1, w2} As shown in FIG. 5 That is, a case of {w1, w2}={+1, +1} represents ACK information and a case of {w1, w2}={+1, −1} represents NACK information. In addition, the relationship between ACK/NACKs and {w1, w2} is not changed regardless of the amount of cyclic shift.
Non-Patent Document 1: R1-074010, Motorola, “Uplink Transmission of CQI and ACK/NAK”, 3GPP TSG RANI #50-bis, Shanghai, China, Oct. 8-12, 2007 Non-Patent Document 2: R1-074097, Samsung, “Multiplexing CQI and ACK/NAK Transmission in E-UTRA UL”, 3GPP TSG RAN WG1 #50bis, Shanghai, China, Oct. 8-12, 2007Non-Patent Document 3: R1-074141, Texas Instruments, “Simultaneous CQI and ACK/NACK Transmission in Uplink”, 3GPP TSG RAN WG1 #50b, Shanghai, China, Oct. 8-12, 2007